1. Field of the Invention
The present invention relates to tool abnormality determination systems that detect abnormalities of a tool such as chipping by monitoring a load during processing by a lathe, etc.
2. Description of Related Art
In a lathe, a load on a tool changes if the cutting edge of the tool is chipped during processing of a workpiece. Specifically, a current value and torque of a motor that moves the tool, and a current value and torque of a motor for a spindle that moves the workpiece fluctuate if the cutting edge of the tool is chipped during processing of the workpiece. The tool abnormality determination systems monitor the load on the tool based on such a change in load. That is, the tool abnormality determination systems compare an actual change in load on the tool with a monitoring range for load monitoring, and determine that there is an abnormality in the tool such as chipping, if the actual load is out of the monitoring range.
Japanese Patent Application Publication No. H07-132440 (JP H07-132440 A) discloses a processing load monitoring method in which sampling data of motor torque is obtained by performing test-cutting a plurality of times and a load monitoring threshold is set based on the sampling data.
In the processing load monitoring method of Japanese Patent Application Publication No. H07-132440 (JP H07-132440 A), however, the threshold or the monitoring range that has been set cannot be changed once processing of a workpiece is started.
Accordingly, if the state of the lathe during setting of the monitoring range is different from that of the lathe during actual processing of the workpiece (when the monitoring range is used), the load tends to be out of the monitoring range even through there is actually no abnormality in the tool.
That is, the state of the lathe before idling is different from that of the lathe after idling. In the state before idling (e.g., upon cold start), lubricant in each part of the lathe has a low temperature and thus has high viscosity, and a ball screw for slide drive has a low temperature. The ball screw therefore has a high preload, and a nut portion does not move smoothly. This reduces mechanical efficiency and thus increases the load on the motors.
On the other hand, in the state after idling, the lubricant in each part of the lathe has a high temperature and thus has low viscosity, and the ball screw for slide drive has a high temperature. The ball screw therefore has a low preload, and the nut portion moves smoothly. This increases mechanical efficiency and thus reduces the load on the motors.
As described above, in the processing load monitoring system method of JP H07-132440 A, sampling data of motor torque is obtained by performing test-cutting a plurality of times, and a load monitoring threshold is set based on the sampling data. Accordingly, if the threshold is set based on the motor torque before idling of the lathe, the load tends to be smaller than the threshold when a workpiece is actually processed after idling.
On the other hand, if the threshold is set based on the motor torque after idling of the lathe, the load tends to be larger than the threshold when the workpiece is actually processed before idling (e.g., the morning after the day the threshold was set).
In the processing load monitoring method of JP H07-132440 A, if the state of the lathe during setting of the monitoring range is different from that of the lathe during actual processing of the workpiece (when the monitoring range is used), the load thus tends to be out of the monitoring range even through there is actually no abnormality in the tool.
Japanese Patent Application Publication No. 2001-150244 (JP 2001-150244 A) also discloses a method of setting a reference value before the start of operation of a gear shaper and permanently storing the reference value (e.g., claim 10 of JP 2001-150244 A), as in the processing load monitoring method of JP H07-132440 A.